1. Field of the Invention
This invention relates to a method of manufacturing glass in which raw material is fed as a batch to a continuous glass-melting tank furnace. The method comprises melting the batch in a melting tank and passing the melt to a refining tank via a submerged throat, heating the melt in the refining tank to de-gas it, delivering molten refined glass to a conditioning tank and there bringing it to a desired working temperature. The invention includes glass manufactured by such a method, and it extends to a continuous glass-melting tank furnace having a melting compartment comprising a tank and superstructure equipped with heating means defining a melting zone for receiving and melting raw batch material, a separate refining compartment also comprising a tank and superstructure equipped with heating means, means defining a throat allowing communication between the lower parts of the melting and refining tanks, and a conditioning tank for receiving melt from the refining tank.
In the manufacture of glass on an industial scale, various problems arise. Among these problems are that of economy regarding heating costs, and that of obtaining a properly refined, bubble-free glass.
It is of course well known that economies of scale are possible, and that a melting furnace of any given size will be most economical when run at its designed production rate. In the remainder of this specification it will be assumed that any furnace referred to is being run at a given, most economical production rate.
2. Discussion of the Prior Art
It is well known that the reactions which take place between the constituents of the raw batch during melting give rise to a considerable amount of surface foam on the melt, and bubbles of gas within the melt. It is also known that in order to refine the glass, that is to say, to ensure the substantially no bubbles remain in the melt which is drawn off for shaping, temperatures are required which are higher than those which are in fact necessary for melting the glass.
Classical glass-melting furnaces have a single tank in which melting and refining take place. Material in the tank is heated from above by burners, and the tank holds a molten mass which at the charging end of the tank is covered by unmelted or only partly melted batch material, and by foam. Somewhere close to the center of the tank there will be a point, the "hot spot" where the melt has its highest temperature and thus least density. Accordingly there will be a "spring zone" of rising currents within the melt. At the walls of the tank, the melt will be at its coolest, and there will be falling currents there. As a result, there will be a return surface current flowing from the spring zone to the charging end of the tank which tends to maintain unmelted batch and foam in the upstream portion or melting zone of the tank so that such batch and foam cannot be drawn off at the downstream end of the refining zone. Such currents will also tend to carry heat energy away to the walls of the tank where it becomes dissipated, and it is not possible to exercise any degree of independent control of the temperatures of the melt in the melting and refining zones of the tank.
In an effort to obtain greater heat economy, proposals have been made to divide the furnace into separate melting and refining tanks. By working in this way, it is possible to exercise a considerable degree of independence in control of the temperatures in the melting and refining tanks. As a result, the melting tank can be run at lower temperatures than are required in classical furnaces with consequent savings in heating costs.
An example of such a plural-tank melting furnace is described in French Patent Specification No. 2,550,523 (Saint-Gobain Vitrage SA). According to the proposals of that specification, glass feeds from the bottom of a melting tank through a throat into the base of a separate refining tank which is shaped as a chimney up which the melt flows in a uniform ascending current while being heated. The melt then passes directly to a conditioning tank where it is brought to a desired working temperature. In fact the principal source of heat both for melting and refining the glass is electric current, though optional burners over the refining chimney are shown.
The cost savings which can be realized using the previously proposed plural-tank melting furnaces are however attainable only at the expense of a lowering of the homogeneity of the glass leaving the furnace. There is also an occasional tendency for the glass to be incompletely degasified. The formation of the refining tank as a relatively deep chimney and the employment of submerged electrical heaters to maintain a strong ascending current of glass in this chimney as proposed in the above mentiioned French Patent Specification No. 2,550,523 would not avoid these disadvantages.